As used herein, the term "fiber" includes fibers of extreme or indefinite length (filaments) and fibers of short length (staple). The term "yarn" refers to a continuous strand of fibers.
"Modification ratio" means the ratio R.sub.1 /R.sub.2 where R.sub.2 is the radius of the largest circle that is wholly within a transverse cross-section of a fiber, and R.sub.1 is the radius of the circle that circumscribes the transverse cross-section.
"Trilobal fiber" means a three-lobed fiber having a modification ratio of at least 1.4.
"Polymer composition" means any specific thermoplastic polymer, copolymer or polymer blend including additives, if any.
Fibers which have a trilobal cross-section are known to be superior in many properties to those having a round cross-section.
It is also known that combining two or more different polymeric components, whether the differences result from differences in additives or in the base polymer itself, produces fibers with improved properties for many end uses. For example, composite polyester fibers which are self-crimpable are disclosed in U.S. Pat. No. 3,671,379 to Evans et al.
Also, U.S. Pat. No. 3,418,200 to Tanner describes a tipped multilobal composite fiber which is readily splittable. U.S. Pat. No. 3,700,544 to Matsui discloses composite sheath/core fibers having improved flexural rigidity. One of the cross-sections disclosed by Matsui is a triangular sheath/core fiber. These patents are merely examples of the variety of effects which can be achieved with multicomponent fibers.
Methods and apparatus for preparing multicomponent fibers are also known. Exemplary apparatus are shown in U.S. Pat. Nos. 3,188,689 to Breen, 3,601,846 to Hudnall, 3,618,166 to Ando et al., 3,672,802 to Matsui et al., 3,709,971 to Shimoda et al., 3,716,317 to Williams, Jr. et al., 4,370,114 to Okamoto et al., 4,406,850 to Hills, and 4,738,607 to Nakajima et al.
As is demonstrated from the previous patents, a great deal of effort has been directed to developing multicomponent fibers, as well as methods and apparatus for producing them. Yet sheath/core trilobal fibers are not presently produced effectively and with sufficient uniformity and efficiency. Also, there has been a lack of the ability to adjust the sheath components in any versatile manner. Thus, there remains a need for a method for producing a sheath/core trilobal fiber where the ratio of sheath to core is relatively accurately controlled as is the composition of the sheath component itself. It is believed that the fibers produced by such a method will find great utility in various applications.